1. Field of the Invention
The present invention relates to a focused ultrasonic piezoelectric actuator and a method of manufacturing the same. More specifically, the present invention relates to a focused ultrasonic piezoelectric actuator for focusing ultrasonic waves using a thickness vibration mode at a MHz frequency band, and a method of manufacturing the same. The focused ultrasonic piezoelectric actuator is easily fabricated by a powder injection molding method and capable of maximizing an ultrasonic focusing efficiency.
2. Discussion of Related Art
Normally, focused ultrasonic piezoelectric actuators used in medical devices are based on optical principles. Since the focused ultrasonic piezoelectric actuator is manufactured by processing a piezoelectric device to have a spherical lens shape, it can focus ultrasonic energy generated by voltage application in a thickness vibration mode into a focal point of curvature, and thereby concentrate the ultrasonic energy. Accordingly, the focused ultrasonic piezoelectric actuator has been used in a surgical procedure for burning away malignant tumor in a human body or burning and decomposing fat and, recently, widely used for the purpose of regeneration of skin by irradiating facial skin with the ultrasonic energy generated from the focused ultrasonic piezoelectric actuator.
Piezoelectric ceramics are materials that convert electric energy into mechanical energy. When piezoelectric ceramics having predetermined thicknesses are resonated in thickness directions, resonant frequencies vary according to the thicknesses to be the same as an applied frequency, and thereby ultrasonic waves are generated. Here, an optical principle may be applied to focus the generated ultrasonic vibration.
FIG. 1 is a view for describing a principle of focusing light in a spherical lens.
As illustrated in FIG. 1, light passing through a spherical lens 5 have different focal lengths according to wavelengths of the light. That is, as the wavelength of the light increases, a focal length of the light decreases. Since the piezoelectric ceramics have maximum displacement values at resonant frequencies thereof, the piezoelectric ceramics are normally operated at the resonant frequencies when used as ultrasonic devices.
FIG. 2 is a view illustrating a direction of vibration displacement generated when a disk-type piezoelectric vibrator is operated in a thickness vibration mode.
As illustrated in FIG. 2, the disk-type piezoelectric vibrator 10 has a maximum displacement in a thickness direction at a resonance frequency thereof since it uses thickness vibration. A piezoelectric material has its own frequency integer, and the frequency integer is represented by the following Formula 1.Nt=fr×t[HZ·m]  [Formula 1]
Nt: frequency integer
fr: first resonant frequency [Hz]
t: thickness of specimen [meter]
A resonant frequency according to a thickness of a specimen is determined by Formula 1. For example, when the frequency integer is 2100, a thickness of a piezoelectric material having a resonant frequency of 2 MHz in a thickness direction may be determined as follows.t [m]=Nt/fr=2100/(2×106)=1.05×10−3 [m]=1.05 [mm]
That is, in order to manufacture an ultrasonic vibrator having the resonant frequency of 2 MHz in a thickness direction, the piezoelectric material needs to be fabricated to have a thickness of 1.05 mm. A piezoelectric device having a preferred resonant frequency may be manufactured according to a frequency integer of a piezoelectric material.
Accordingly, a piezoelectric device may be designed to have a preferred focal length by using a principle of a spherical lens and a frequency integer in a thickness vibration mode. That is, a focused ultrasonic device having a preferred frequency and focal length may be fabricated by manufacturing a piezoelectric device to have a thickness of the preferred frequency for generating ultrasonic waves and to have a hemispherical shape. Normally, ultrasonic waves are focused in an area corresponding to a radius of curvature of a dome-shaped piezoelectric device.
FIG. 3(a) is a view illustrating a bulk-type piezoelectric device, and FIG. 3(b) is a view illustrating a state in which the bulk-type piezoelectric device of FIG. 3(a) is processed to have a preferred thickness.
Conventionally, a bulk-type piezoelectric device 10 as illustrated in FIG. 3(a) is manufactured, and then the bulk-type piezoelectric device 10 is processed to have a preferred thickness and radius of curvature as illustrated in FIG. 3(b) by using a grinding machine as a lens-processing method.
Accordingly, a large amount of the materials may be consumed and cracks may easily occur since the piezoelectric ceramics are brittle during processing, resulting in rise of manufacturing costs in a mass production process.
Normally, dome-shaped piezoelectric ultrasonic devices used for skin regeneration mainly have a resonant frequency of 4 MHz or 7 MHz. According to Formula 1, the dome-shaped piezoelectric ultrasonic devices are subjected to rounding processing to have thicknesses of 0.525 mm and 0.3 mm, respectively. Thus, as the resonant frequency increases, the thickness of the dome-shaped piezoelectric ultrasonic device is reduced to 0.3 mm or less. Accordingly, internal stress caused by the processing of a brittle piezoelectric material is increasingly accumulated and risk of breakage increases while the dome-shaped piezoelectric ultrasonic device vibrates by a strong AC electric field of 250 Vrms/mm.
In addition to such problems due to mechanical processing, spurious vibrations generated in edge portions of the conventional dome-shaped focused ultrasonic device may prevent focusing of ultrasonic waves and thereby reduce an ultrasonic focusing efficiency.
As a reference, a piezoelectric-piezoelectric ceramic actuator having opposite polarization directions is disclosed in Korea Publication Patent No. 10-2003-0095638 (published on Dec. 24, 2003).